The possibility of rapid sea level rise

A recent study in Nature examines data from corals in Mexico and concludes that very rapid sea level rise took place during the Eemian period – a previous interglacial where temperatures were about 2°C warmer than they are at present. Sea level rise during the period is estimated at four to six metres, as the result of ice sheets collapsing. On the basis of this data, geoscientist Paul Blanchon concludes that: “a sudden, catastrophic increase of more than 5 centimetres per year over a 50-year stretch is possible.” Obviously, that is a much more rapid and dramatic increase than the one included in the fourth report of the IPCC.

Fifty miles off the coast of New Jersey, a team of scientists and engineers are drilling the ocean floor for clues about sea levels and global warming. Slate V contributor John Pavlus visited the floating rig.

“During my [2003 White House] presentation, I argued that paleoclimate records provide guidance for the level of warming that would be dangerous from the perspective of sea level change. Specifically, I pointed out that prior warmer interglacial periods such as the Eemian were only about 1 degree Celsius warmer than today, on global average, yet sea level was four to six meters higher than today.”

Fossilised coral reefs in the Great Barrier Reef could help scientists understand how sea levels have changed over the past 20,000 years.

An international team of researchers will spend 45 days at sea, gathering core samples from about 40 sites.

Described as the “trees of the sea”, coral have growth rings that show seasonal variations.

Researchers say the samples will also shed light on past sea temperatures, as well as other changes to the reef.

Alan Stevenson, team leader of marine geology at the British Geological Survey (BGS), which is involved in the project, said the fossilised corals’ annual growth rings provided an insight to conditions under waves.

“We can then analyse those rings to build up a very detailed picture of what the ocean was like when they were forming, including temperature and salinity.

“In its latest report, the IPCC has predicted up to 59 cm of sea level rise by the end of this century. But realclimate soon revealed a few problems.

First, although the temperature scenarios of IPCC project a maximum warming of 6.4 ºC (Table SPM3), the upper limit of sea level rise has been computed for a warming of only 5.2 ºC – which reduced the estimate by about 15 cm. Second, the IPCC chose to compute sea level rise up to the year 2095 rather than 2100 – just to cut off another 5 cm. Worse, the IPCC report shows that over the past 40 years, sea level has in fact risen 50% more than predicted by its models – yet these same models are used uncorrected to predict the future! And finally, the future projections assume that the Antarctic ice sheet gains mass, thus lowering sea level, rather at odds with past ice sheet behaviour.**

Some scientists within IPCC warned early that all this could lead to a credibility problem, but the IPCC decided to go ahead anyway.

Right now, we’re headed towards an ice-free planet. That takes us through the Eemian interglacial period of about 130,000 years ago when sea levels were 15 to 20 feet higher, when temperatures had been thought to be about 1°C warmer than today. Then we go back to the “early Pliocene, when sea level was about 25 m [82 feet] higher than today,” as NASA’s James Hansen and Makiko Sato explain in a new draft paper, “Paleoclimate Implications for Human-Made Climate Change.”

The question is how much warmer was it in the Eemian and early Pliocene than today — and how fast can the great ice sheets disintegrate?

We already know we’re at CO2 levels that risk catastrophe if they are sustained or exceeded for any extended period of time (see Science: CO2 levels haven’t been this high for 15 million years, when it was 5° to 10°F warmer and seas were 75 to 120 feet higher).

Hansen and Sato go further, saying we’re actually at or very near the highest temperatures of the current Holocene interglacial — the last 12,000 years of relatively stable climate that has made modern civilization possible.

On January 7th India’s Border Security Force (BSF) shot dead Mr Nur Islam’s 15-year-old Felani, at an illegal crossing into Bangladesh from the Indian state of West Bengal. Felani’s body hung from the barbed-wired fence for five hours. Then the Indians took her down, tied her hands and feet to a bamboo pole, and carried her away. Her body was handed over the next day and buried in the yard at home.

The BSF kills with such impunity along India’s 4,100-kilometre (2,550-mile) border with Bangladesh that one local journalist wonders what the story is about. According to Human Rights Watch, India’s force has killed almost 1,000 Bangladeshis over the past ten years. That implies a shooting every four days. The death toll between two democracies dwarfs the number killed attempting to cross the inner German border during the cold war. Mr Islam does not know why things went wrong. Like many from Kurigram, a desperately poor district in northern Bangladesh, he had made the crossing countless times before. He had paid 3,000 rupees ($65) to traffickers on the Indian side.

What happens in the Arctic doesn’t stay in the Arctic, as you recognise (“Polar bare”, April 29th). What we have seen to date is just the tip of the iceberg. The rising sea level, centimetre by centimetre, is inexorably moving shorelines, laying waste to infrastructure and wreaking havoc on property values. Around the world, too many are failing to plan for the foreseeable consequences.

The sea is rising, at least a metre within the lifetime of today’s youth and perhaps over three metres if climate mitigation is not pursued aggressively. After 5,000 years of stability, we need to develop long-term pragmatic plans to cope with the disruption. This means investing to adapt our infrastructure, from bridge heights to water treatment facilities to public transport.